Stereoisomerism. Isomers (q.v.) of organic compounds, such as of simple sugars, having the same molecular (q.v.) composition but differing in the spatial positions of their atoms (q.v.) or groups of atoms that act as radicals (q.v.). This gives rise to compounds with molecular structures that are mirror images of each other and exhibit dextrorotation (q.v.) and levorotation (q.v.) in a polarimeter (q.v.).